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Force control
Definition
Force control regulates the force or torque a machine applies while interacting with an object or its environment. In robotics, it helps manage contact, rather than only commanding the robot to reach a particular position.
What it means
A robot polishing a surface needs to move along it while maintaining appropriate contact pressure. Position commands alone may produce too much or too little force if the surface position differs slightly from the model.
Force control addresses that interaction. Depending on the design, the controller uses measured force, a model of the robot or both to adjust its commands.
The topic belongs to control engineering and does not inherently require machine learning. AI may assist perception, planning or adaptation, while the force-control loop remains a conventional engineered controller.
For business readers, the value is consistent contact behavior in tasks such as assembly, finishing and handling. Performance depends on the complete robot, tooling, sensing and material interaction, not simply on adding a feature labeled force control.
How it works
The engineer defines the desired contact force or wrench, where wrench means the combined force and torque description. Sensors or models provide information about the current interaction.
The controller compares the desired and measured behavior and adjusts actuator commands. Robot geometry and dynamics affect how joint forces or torques translate into forces at the tool.
Some tasks combine motion control in unconstrained directions with force control in constrained directions. For example, a tool may move along a surface while regulating its normal contact force.
Design must account for sensor noise, delays, contact stiffness and actuator limits. Tuning and validation use the actual operating conditions, because a controller that behaves well against a soft material may respond differently against a rigid surface.
Three practical examples
- Surface finishing: maintain a controlled contact load while a tool follows the workpiece.
- Assembly: manage contact during an insertion task when small alignment differences occur.
- Handling: regulate gripping interaction for objects whose shape or compliance varies within the intended application.
Think of it this way
Think of wiping a tabletop while adjusting hand pressure as the cloth moves over uneven spots. The motion path and the pressure are related but distinct aspects of the task.
The analogy has a limit. A robot lacks the full tactile judgment of a person unless suitable sensing and control have been engineered. Desired force values and response limits must be established for the particular equipment and material.
Worked example
Interpreting a force error
Suppose an invented laboratory example has a desired contact force of ten newtons and a measured force of eight newtons. The error defined as desired minus measured is two newtons.
A controller can use that error as part of its command calculation. The appropriate actuator change depends on the robot, contact geometry and control law; adding two directly to an arbitrary motor command would not generally be meaningful.
If the sensor later reads 12 newtons, the same error definition gives minus two newtons, signaling that the measured force exceeds the target.
These values illustrate feedback terminology only. They are not operating settings for an actual robot. Real commissioning requires equipment-specific engineering, limits and tests that reflect the contact task.
Illustrative case study
A fictional finishing company programs a robot to follow a fixed path over panels. Small panel-height variations create inconsistent contact, so some areas receive too little treatment and others too much.
The engineering team evaluates a suitable force-controlled process with representative panels and tooling. It measures finish quality, contact behavior and cycle time rather than assuming a force sensor alone resolves the problem.
The lesson is that control objectives should match the physical interaction. Accurate movement through free space does not automatically produce consistent work when the tool presses against a variable surface.
Common misconceptions and mistakes
- Force control is simply position control with a different name. They regulate different aspects of interaction, although a system can combine them.
- A force sensor by itself controls force. Sensing supplies information; a suitable controller and actuation system must use it.
- Force control makes every contact task safe. Performance and limits depend on the full engineered system and its validated application.
Business use
Force control can improve process consistency when contact quality matters. It is relevant to robotics procurement, assembly design and automation of tasks involving variable surfaces or objects.
Ask suppliers to demonstrate representative materials, tolerances and contact conditions. Compare quality and failure recovery as well as nominal cycle time. Clarify which directions are motion-controlled, which are force-controlled and how the system handles lost contact or unexpected resistance.
People also ask
Does force control always need a tool-mounted sensor?
Not always. Some approaches use models or other measurements, while direct force feedback can improve information about contact.
Is it the same as impedance control?
They are related, but impedance control specifies a relationship between motion and force rather than only a direct force target.
Can a robot control motion and force together?
Yes, with a suitable formulation and attention to constrained and unconstrained directions.
Check your understanding
A polishing robot follows its programmed position accurately but presses inconsistently on uneven panels. Which missing consideration does this suggest?
Answer: Contact force must be considered alongside motion. Accurate position tracking alone does not establish consistent interaction with a varying surface.
Related terms
- Feedback
- robot control
- force-torque sensor
- impedance control
- hybrid motion-force control
- end effector
Learn next
Read feedback control and impedance control. Describe one contact task in terms of required motion, desired interaction and the measurements needed to verify both.
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Sources
Lynch and Park, Modern Robotics, Force Control: https://modernrobotics.northwestern.edu/nu-gm-book-resource/11-5-force-control/. Primary author-hosted explanation consulted. Force-error arithmetic and finishing scenario are original, illustrative examples.
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